906 THE GEOLOGY OF MINNESOTA. 



[Muscovadyte. Gabbro. 



A characteristic, gray muscovadyte. 



Mil-. While the general grain is fine, the structure is decidedly ophitic. The 

 migite has a violet tint. Besides the feldspars the rock contains some quartz and 

 biotite. One (thick) section. 



Age. Cabotian. N. H. w. 



Remark. Owing to the importance of the ophitic structure in this rock, another 

 slide was prepared. In this the characteristic granular structure is well displayed, 

 differing from the foregoing. The augite has the lamellation of diallage, and, while 

 it occasionally surrounds roundish feldspars, the augite thus surrounding the feld- 

 spars is not of the same orientation, but is divided between two or four or more 

 orientations. It is but rare that a single grain of augite, having a single orientation, 

 wholly surrounds a single roundish feldspar, yet there are a few such instances. 

 This slide does not show the normal ophitic structure of the foregoing, since the 

 feldspars thus included do not possess their crystal forms. It is possible, therefore, 

 that the former slide, by some mistake in handling, was not made from the rock No. 

 847G. One section. N. H. w. 



No. 854G. GABBRO? (witfti/n'irtz). 



S. E. J4 S. W. % sec. 12, T. 64-6, southwest shore of a small lake west of Little Saganaga lake. 

 Ref. Annual Report, xxi, page 65. 



Meg. A pinkish-gray syenitic rock. 



Mic. The feldspars appear to be both orthoclase and plagioclase, but they are 

 crowded with muscovite, calcite, iron ores, and perhaps other products of alteration, 

 and are closely associated with quartz. Within the quartz, and elsewhere, is petmine. 

 Quartz embraces the altered feldspars in an ophitic manner, yet in the slide it is also 

 in isolated patches that extinguish in common. There is no olivine nor augite. If 

 these ever existed in this rock, which is doubtful, they are now represented by the 

 pennine and iron ores. One section. 



Age. Cabotian. 



Remark. It may be considered very questionable whether this rock ever was 

 a gabbro. It is now more nearly a granite, and seems to have resulted from the 

 transformation of an " intermediate " clastic rock. N. H. w. 



No. 854aG. GABBRO (with qiuirtz). 



Same place as the last. 



Ref. Annual Report, xxi, page 66. 



Meg. A coarse gabbroid rock, with some pinkish feldspar and some quartz. 



Mic. Essentially like No. 854G, but having more of the structure of gabbro 

 while lacking its minerals. There are areas that are now occupied by biotite (or 

 biotite and chlorite) which appear to have been once occupied by augite, inasmuch 



